Selective transplantation of purified erythrocyte precursors will be evaluated as a method to repopulate erythrocytopoiesis, when either precursor cells are depleted or a genetically inherited abnormal erythrocyte population exists. The proposed study is designed to accomplish the following specific objectives: 1) Determine the number of purified erythrocyte precursor elements that may be obtained from fetal livers at varying periods of gestation. 2) Evaluate both canine and human fetal liver erythrocyte precursors for the emergence and maturation of histocompatibility antigens and the presence of potentially immunocompetent cells. 3) Establish a model test system to evaluate the feasibility of employing fetal liver erythrocyte precursors for selective transplantation repopulation of erythrocytes. 4) Investigate the relative roles of erythrocyte blood group isoantigens and histocompatibility antigens in the rejection of erythrocyte precursors and mature cells. 5) Determine if a purified suspension of transplanted erythrocyte precursors can initiate graft-vs.-host disease. 6) Evaluate the capability of antithymocyte antisera to reduce both immune rejection and a graft-vs.-host syndrome in animals undergoing selective erythrocyte transplantation.